That seems extremely out of order though, even if someone else is paying, he's still going out and enjoying himself. Have you tried sitting him down and explaining to him that going to a slimming club really isn't the same as going out with friends?
Yeah I don't blame you, go out and have a ball. If your sisters paying for you then he can't complain anyway really, even with your night out to fat club on Monday
My boyfriend is sort of a pain the other way round, he really doesn't mind me going out, I felt really guilty last month as I ended up going on a mini hol with my mum, only one night in Castleton, she invited me because he was on a course for two days and she thought it was overnight, but it wasn't, he was home on the evening. He really didn't care, but the following weekend he was arranging for us to go do something with his friend, his friends girlfriend was busy so I said rather then me tagging along how about he went to his friends on his own because they could talk and do their thing without worrying about whether I was interested in what they were doing or not, anyway, I got a text about 10 O'clock saying he missed me and was coming home :/